1. Field of the Invention
The present disclosure relates to an apparatus for estimating capacitance of a DC-link capacitor in an inverter.
2. Background of the Invention
A multi-level medium-voltage inverter is an inverter in which an effective value of an input line voltage is 600V or higher. The multi-level medium-voltage inverter outputs an output phase voltage having multiple stages (or multiple levels). A medium-voltage inverter is used to drive a motor having large capacity ranging from hundreds of kW to tens of MW, and mainly used in the fields such as fans, pumps, compressors, traction, hoist, conveyor, and the like.
Among multi-level medium-voltage inverters, a serial-type H-bridge inverter having a modular structure is easily extendable, so it is commonly used as a medium-voltage inverter. The serial type H-bridge inverter includes a DC link capacitor having large capacitance in every unit power cell, and here, the DC link capacitor, among constituent components of a power conversion circuit, is relatively easily broken.
FIG. 1 is a view illustrating a related art serial-type H-bridge medium voltage inverter.
As illustrated in FIG. 1, the related art medium voltage inverter 100 receives a voltage having a line voltage effective value equal to or higher than 600V from an input power source 200, converts it into a 3-phase voltage, and outputs the same to a motor 300. The motor 300 is a 3-phase motor having a medium voltage.
The medium voltage inverter 100 includes a phase replacement transformer 110 and a plurality of unit power cells 120a to 120f. 
The phase replacement transformer 110 electrically insulates the input power source and the medium voltage inverter 100, reduces harmonics from an input terminal, and provides input 3-phase power to the respective unit power cells 120a to 120f. 
Upon receiving power from the phase replacement transformer 110, the unit power cells 120a to 120f outputs a phase voltage of the motor 300. The respective unit power cells are divided into three groups. In FIG. 1, a power cell A1 120a and a power cell A2 120b are connected in series to synthesize an a-phase voltage of the motor 300. A power cell B1 120c and a power cell B2 120d are electrically connected to synthesize a b-phase voltage. A power cell C1 120e and a power cell C2 120f are electrically connected to synthesize a c-phase voltage. The synthesized b-phase voltage and a-phase voltage has a phase difference of 120 degrees, and the c-phase voltage and the b-phase voltage also have a phase difference of 120 degrees.
FIG. 2 is a view illustrating a detailed configuration of a unit power cell of FIG. 1.
As illustrated in FIG. 2, a unit power cell 120 of a general medium voltage inverter includes a rectifying unit 121, a DC link capacitor 122, an inverter unit 123, a voltage sensor 124, a current sensor 125, a driving unit 126, and a controller 127.
The rectifying unit 121 receives an electrically insulated AC voltage from the phase replacement transformer 110 and converts it into a DC voltage. The rectifying unit 121 generally includes a plurality of diodes, and the rectified voltage is determined by a difference between input power and output power of the rectifying unit 121 and capacitance of the DC link capacitor 122.
The DC link capacitor 122 compensates for a power difference between the rectifying unit 121 and the inverter unit 123, and the voltage sensor 124 measures a voltage from the DC link capacitor 122.
The inverter unit 123 is a single phase full bridge inverter and synthesizes output from a DC link voltage to the motor 300 through a power switch.
The current sensor 125 measures an output current from the inverter unit 123.
The driving unit 126 independently transmits a driving signal to each unit power cell. The driving unit 126 receives a voltage reference from the controller 127, generates a gating signal for determining a switching state of the inverter unit 123, and provides a state of a unit power cell to the controller 127.
The controller 127 applies a voltage reference Vref to each unit power cell 120 and determines a sequence of an overall system. The controller 127 may determine a voltage reference applied to each unit power cell 120 according to a user command and setting.
According to the voltage reference from the controller 127, the driving unit 126 may determine a gating signal with respect to the voltage reference in consideration of the DC link voltage Vdc. Also, when the driving unit 126 determines that the DC link capacitor has an error according to the output current Iout and the DC link voltage Vdc, the driving unit 126 may stop generating of the gating signal.
The DC link capacitor used to smooth the DC link voltage has a short lifespan and high fault generation frequency, relative to the other elements, significantly affecting reliability of the inverter. In order to recognize a fault state of the DC link capacitor 122, conventionally, an additional device is required, and also, although an additional device used, it is used merely in a particular operational state such as a power separation state, making it impossible to measure a state thereof in real time.